Heating unit and method of making the same

Abstract

A heating unit includes an AlN substrate having a main surface on which an elongated heat-generating resistor is provided. A protection layer is formed on the main surface of the substrate for the heat-generating resistor. The protection layer includes a first cover layer covering the heat-generating resistor and a second cover layer covering the first cover layer. The first cover layer is made of crystallized or semi-crystallized glass having a higher crystallization temperature by at least 50° C. than the softening point of the glass. The second cover layer is made of non-crystalline glass.

1. A heating unit comprising:
 an AlN substrate including a main surface; 
 an elongated heat-generating resistor provided on the main surface of the AlN substrate; and 
 a protection layer for the heat-generating resistor; 
 wherein the protection layer includes a first cover layer covering the heat-generating resistor, a second cover layer covering the first cover layer, and a third cover layer that covers at least part of an exposed region of the main surface where the first cover layer is not provided, 
 wherein the first cover layer is made of crystallized or semi-crystallized glass having a higher crystallization temperature by at least 50° C. than the glass softening point of the crystallized or semi-crystallized glass, the second cover layer being made of non-crystalline glass, 
 wherein the third cover layer is made of non-crystalline glass higher in glass softening point than the non-crystalline glass constituting the second cover layer, and wherein the second cover layer is provided on the first cover layer and at least part of the third cover layer. 
 
     
     
       2. A method of making a heating unit, the method comprising the steps of:
 forming, by sintering, an elongated heat-generating resistor on an AlN substrate; 
 forming, by sintering ,a first cover layer to cover the heat-generating resistor; 
 forming, by sintering, a third cover layer on the AlN substrate; and 
 forming, by sintering, a second cover layer to cover the first cover layer and at least part of the third cover layer; 
 wherein the first cover layer is made of crystallized or semi-crystallized glass having a higher crystallization temperature by at least 50° C. than the glass softening point of the crystallized or semi-crystallized glass, the sintering of the first cover layer being performed at a higher crystallization temperature by 50 to 70° C. than the glass softening point of the crystallized or semi-crystallized glass, 
 wherein the second cover layer is made of non-crystalline glass, the sintering of the second cover layer being performed at a temperature higher by at most 100° C. than the glass softening point of the non-crystalline glass. 
 
     
     
       3. The method according to  claim 2 , wherein the glass softening point of the crystallized or semi-crystallized glass constituting the first cover layer is no lower than 740° C., the sintering temperature of the first cover layer being in a range of 800 to 850° C. 
     
     
       4. The method according to  claim 2 , wherein the glass softening point of the non-crystalline glass constituting the second cover layer is no lower than 700° C., the sintering temperature of the second cover layer being in a range of 800 to 850° C. 
     
     
       5. The method according to  claim 2 , wherein the third cover layer is made of a non-crystalline glass higher in glass softening point than the non-crystalline glass constituting the second cover layer, and wherein the sintering of the third cover layer is performed at a temperature higher by at most 30° C. than the glass softening point of the non-crystalline glass constituting the third cover layer.